It has long been known that a transistor may be stabilized, as shown in FIG. 1, by connecting a resistor R between the base B and the emitter E of the transistor. It has also been known to form the resistor R by a diffusion in the same semiconductor body in which the transistor itself is formed.
In some transistors, a so-called "floating" emitter of the same conductivity type as the ordinary emitter of the transistor is formed surrounding the ordinary emitter in order to enhance the ASO (Area of Safe Operation) characteristic of the transistor. FIG. 2 shows such a transistor provided both with a stabilizing resistor and a floating emitter.
In this transistor, a P-type base region 2 is first formed by diffusion in an N-type silicon wafer 1. Next, an N-type emitter region 3 is formed in the base region 2. A ring-like region 4 surrounding the emitter region 3 forms the floating emitter. An elongated resistive region 5, which acts as the stabilizing resistor R, is formed in the same diffusion step used to form the ordinary emitter region. The resistive region 5 is connected between the base region 2 and the emitter region 3 through respective metal electrodes 6 and 7.
Since the impedance to the emitter 3 from the part of the resistive region 5 closest to the emitter 3 is small, and as that part of the resistive region 5 is at substantially the same potential as the emitter region 3, this part of the resistive region 5 causes an unwanted transistor action to occur. Hence, there is a problem with this device in that secondary breakdown can occur in this area so as to cancel out the advantageous effects provided by the floating emitter.
To overcome this problem, there has been proposed an arrangement, as shown in FIG. 3, in which resistive region 5 is entirely surrounded by an N-type region 8. This approach, however, has not been found to be completely acceptable for various reasons, including the fact that a relatively large amount of wafer surface area is required.
Accordingly, it is an object of the present invention to provide a transistor in which the above-mentioned secondary breakdown due to the presence of the resistive region is eliminated.
It is also an object of this invention to define a transistor structure wherein the base electrode is positioned close to the emitter end of a resistive region.